Comparing Drawings:

The Good

  

The "Needs Improvement"
Good Drawing - note correct perspective and convincing illusion of volume. Also note the assertive, experienced use of line. Created by a modern artist. Mediocre Drawing - note problems with perspective. There are sketchy, "back and forth" lines suggesting inexperience.
   
Good Drawing - no wonder, it's done by Rafael. Pay attention to the depth that is evident when looking at it. There's a lot of attention to perspective. Mediocre Drawing - the technique suggests no depth or dimension, looks amateurish.
   
 
Good Drawing - Michelangelo this time. See how rudimentary shading added helps us see the shape of the object.